Scott called me out on my disparaging remarks about certain objects in the Messier catalog, and a couple of younger readers aren't sure what we are talking about. So here's a teeny bit of history. I promise it won't hurt, even though it involves French people.
Astronomy a few hundred years ago was a very different science than it is today. For 2000 years, people had studied the sky, but for reasons that were turning out to be pointless. Then along comes the telescope, and in the early 1600s, shows that really everything we knew was wrong anyway. By the end of that century, most of the resistance to new ideas in science was going away, and astronomy was ripe for being re-invented as a new science.
The catch was, "science" as we think of it now was also just being invented...or at least the ideas that mathematical models could be compared against actual empirical studies, and the results used to check each other. The lads who started the Royal Society were in no small part for eventually deciding on just what "science" and the "scientific method" were, all around this time. But in Europe, Universities and States didn't really see any reason to fund "research" and other messy things like that, and since it could often be expensive-and dangerous-it fell to rich guys with not much work to do to start the ball rolling. Being one of the Gentry provided plenty of money for stuff, and the inbreeding that went with it no doubt helped with doing the dangerous parts.
So early "modern astronomy" really got rolling in the early 18th century, and most of the work was done by people who were experts in other fields, who could simply afford to build a telescope. Many of them were great names in science, but generally not for their work in astronomy; we tend to remember them for things that actually took only a small part of their time. Edmund Halley was a geologist and mathematician; today we remember him just for making an almost offhand prediction about a comet that had popped around a few times before, for one example.
In fact, what a lot of these guys were into was comets. There were no major sky catalogs yet, and making one was certainly too much work for someone who had other real work to do...but those comets, man, those were really exciting. The skies were essentially unchanging after all, or at least predictable once Newton's (mathematical) model of where to look for planets was borne out. So there was little need to study it, it was felt. But comets-they broke the rules-they showed up randomly, they changed their shape and appearance, etc. Plus, you could get some real nice pats on the back from fellow philosophers (as scientists were called then) by finding one first.
So, now we have a bevy of well-heeled smart fellows scanning the skies for comets. When a comet is far away, it looks like a slightly fuzzy star. As it grows closer, it gets fuzzier. So they looked for fuzzy stuff. And found a lot of fuzzy things that were NOT comets-they stayed in the same place. And different guys were spending time discovering the SAME non-comets.
Charles Messier, in France,after much work, published his first list of Messier Objects in 1771. He did not discover all of the items in the list; he was a compiler. Over the centuries, others have added to his original list, since there are letters and such that show he had been told of other items as well, but after 100 or so items he seems to have lost some interest. The last couple of items in his list were actually added in the 1940s. The list is not particularly well-ordered, it often skips around the sky in a charming manner, and even skips some items right next to others that were included (such as in the Leo Triplet of galaxies; NGC 3628 is certainly visible to anyone who could see M65 and M66.)
The singular purpose of this list of this list was to allow the astronomer at his telescope the ability to determine if that faint splodge was something new, or something he could ignore. It was a handy thing. It was a list of items that were nebulous, or cloudy...things that could not be focused sharply, since that was what the comet hunters were searching for.
The notion of "science" was certainly developing in the century that followed, and in the 1880s a new catalog of celestial items was created, called the New General Catalog. It included all of the Messier objects, so those all have an NGC number in addition to their Messier number. It's intention was to be a proper catalog of celestial objects, rather than a helpful guide for just one type of stargazer. Other catalogs have been made since, but those first two, the Messier and New General Catalog, contain basically everything that can be found by casual backyard telescopists. In truth, most of us, myself included, have much better telescopes than what was round during Messier's day...several of the Messier objects are very obviously just a handful of stars to us, and it's hard to imagine them being seen as nebulous at all.
One sort of object like that is called an open cluster-from a few dozen to a few hundred stars, clumped loosely together, and with binoculars especially they can be very beautiful. Many can be seen by eye from a nice dark spot. I tend to skip them when using my scope, since they often make for a pretty boring photo. Not always, of course, and some of the open clusters are pretty things indeed.But to answer the original questions, the items in the Messier Catalog are all good targets for scopes of the sizes people tend to have in their backyards, between 4" and 12" of aperture. So many of the things I will post pictures of are called "Mxx".
Ironically, as far back as 1654, an Italian astronomer named Giovanni Battista Hodierna had compiled a list of objects that could be confused for comets, more than a century before Messier. As often happens in the history of science, it was totally ignored.
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